Certain 1H-pyrano3',4':6,7!indolizino1,2-b!quinolinones are known to have cytotoxic and antiviral activity. Camptothecin is an example of such compounds. It is a water-insoluble, cytotoxic alkaloid produced by Camptotheca acuminata trees indigenous to China and Nothapodytes foetida trees indigenous to India. Camptothecin and its close congeners are known to inhibit eukaryotic topoisomerase I. In fact, the cytotoxic and antitumor activity of camptothecin and its close congeners results from inhibition of eukaryotic topoisomerase I (Cancer Res. 1988, 48, 1722; Molec. Pharmacol. 1988, 34, 755). Compounds that are related in structure to camptothecin but do not inhibit eukaryotic topoisomerase I are not cytotoxic to mammalian cells and have no antitumor activity (J. Med. Chem. 1988, 32, 715; Cancer Res. 1989, 49, 1465; Cancer Res. 1989, 49, 4358).
A number of investigators have shown that camptothecin possesses antiviral activity. However, although camptothecin has demonstrated antiviral activity in in vitro tissue culture systems, camptothecin and its close analogs that have a hydroxylactone moiety cannot be considered as useful in vivo antiviral agents because they undesirably inhibit mammalian topoisomerase I, as well as host cell DNA replication, and are cytotoxic to mammalian cells. Furthermore, camptothecin is not an attractive candidate for drug development as an antiviral agent because of unacceptable dose-limiting toxicity, unpredictable toxicity, and poor aqueous solubility, and/or unacceptable shelf life stability.
There is a need for new antiviral agents. Substituted indolizino1,2-b!quinolinones that lack the a-hydroxylactone moiety of camptothecin have been shown to be non-cytotoxic to mammalian cells and to lack antitumor activity (Ann. Rev. Pharmcol. Toxicol. 1977, 17, 117; J. Med. Chem. 1989, 32, 715). This is because these compounds do not contain the essential structural features required to inhibit eukaryotic topoisomerase I. However, recently we have found that certain substituted indolizino1,2-b!quinolinones lacking the a-hydroxylactone moiety do have antiviral activity but not the undesirable cytotoxicity of camptothecin. Thus, such substituted indolizino1,2-b!quinolinones are useful for treating viral infections.